This swim fin relates to the type of swim fin in which a rigid blade is spaced from the foot pocket and the center portion of the leading edge of the rigid blade is pivotally attached to two beams which are resilient to some extent and which project from the foot pocket.
In the swim fin of prior patent 4,767,368, the resisting force required to support the rigid blade in an angular direction at the operating blade angle during swimming is provided by an extension spring which is designed for a swim fin operating at cruising speed.
One of the problems with such swim fin is that when the swimmer accelerates or when the swimmer starts out from a stationary position, the swimmer has to kick harder causing over stressing of the extension springs, excessive deflection of the blade and some loss of thrust.
Other problems with the extension springs are: they generate drag and they are expensive to use in a swim fin.